(a) This invention relates to a collector made of a polyolefinic fiber having an amidoxime group and a hydrophilic group and which is capable of efficient adsorptive recovery of useful metals such as uranium, vanadium, cobalt and titanium that occur dissolved in small quantities in seawater. The invention also relates to a process for producing the collector.
Seawater has various metals (see Table 1) dissolved in it and the present invention aims at recovering these dissolved metals by adsorption using a collector.
(b) The invention relates to a collector that is produced by introducing an amidoxime group, either alone or in combination with a hydrophilic group, into side chains grafted to a polyolefinic fiber substrate and which needs only to be anchored in seawater to accomplish efficient recovery of useful metals such as vanadium, cobalt, uranium and titanium that are dissolved in the seawater. The invention also relates to a cassette of such collectors and a method of collecting the above-mentioned useful metals from seawater using the cassette.
To produce the collector of the invention, a polymerizable monomer such as acrylonitrile (CH2xe2x95x90CHCN) that contains a cyan group (xe2x80x94CN) is grafted onto a polyolefinic fiber substrate by radiation-initiated graft polymerization so as to form grafted side chains and the cyan groups in these side chains are reacted with hydroxylamine (NH2OH) or the like to be converted to amidoxime groups.
A plurality of the thus produced collectors may be sandwiched between nets and a plurality of the resulting assemblies are stacked in position at suitable spacings to construct a collector cassette. The cassette may be placed in a number of cages that are anchored in seawater to recover useful dissolved metals from it by adsorption.
(a) Conventionally, amidoxime groups are introduced into a polymer structure in accordance with the following scheme (1) by reacting the cyano group (xe2x80x94CN) with hydroxylamine (NH2OH): 
To synthesize a satisfactory amidoxime resin by introducing amidoxime groups into a polymer structure, the introduction of amidoxime groups into substrates typically made of the general-purpose polyacrylic fiber or polyacrylic beads produced by emulsion polymerization. However, these acrylic resins have suffered from deterioration in skeletal strength of the polymer on account of the introduction of hydrophilic amidoxime groups into the cyano groups in the polymer skeleton. With a view to preventing this problem, a review has been made to form crosslinks in the polymer structure. In fact, however, the increase in the degree of crosslinking is accompanied by a decrease in the rate of metal adsorption and this tradeoff has been an obstacle to the solution of the problem.
It is known that a collector that is capable of selective adsorptive recovery of dissolved metals from seawater can be produced by grafting acrylonitrile onto a polyethylene fiber under exposure to radiation and then reacting it with hydroxylamine to introduce amidoxime groups.
It is also known that a selective adsorbent of uranium dissolved in seawater can be produced from a substrate of a desired shape that is made of an inorganic material, an organic material or a composite thereof and into which both an amidoxime group and a hydrophilic group are introduced by radiation-initiated graft polymerization (see Japanese Patent Publication No. 58775/1987) filed by one of present inventors).
Under the circumstances, there has been a pressing need to improve the existing collectors and develop a material that is strong enough to withstand prolonged exposure to hostile weather conditions in ocean and which maintains high performance in collecting vanadium, uranium and other useful metals in seawater.
(b) In seawater, vanadium, uranium and many other rare metals that scarcely occur in Japan are contained dissolved but their concentrations are extremely low, only about 1.9 mg of vanadium per ton of seawater and about 3.3 mg of uranium.
Heretofore, uranium has been recovered from seawater by the following methods using an adsorbent; seawater is brought into contact with the particles of titanic acid to adsorb uranium from the seawater and fine air bubbles are attached to the particles of titanic acid, which are then floated on the seawater and separated therefrom to recover the uranium (Japanese Patent Public Disclosure No. 61018/1979); calcium or carbonate ions are removed from seawater before uranium-in the seawater is recovered by adsorption onto a hydrous metal oxide adsorbent (Japanese Patent Public Disclosure No. 79111/1979); a collector produced by reacting a polyethyleneimine derivative with hydroxylamine is used to achieve adsorptive recovery of metal ions dissolved in seawater (Japanese Patent Public Disclosure No. 48725/1987); and using a kalixarene derivative to recover uranium in seawater by adsorption (Japanese Patent Public Disclosure No. 136242/1987).
Dissolved metals can also be recovered using chelate resins and conventional methods based on this approach include the following: a specified group is introduced into a chloromethylated crosslinked polystyrene, which is then reacted with hydroxylamine to produce an adsorbent resin that is used to recover dissolved metals from seawater by adsorption (Japanese Patent Public Disclosure No. 84907/1984); a chelate resin having malonyl dihydroxamate residue is used as an adsorbent to recover dissolved metals by adsorption (Japanese Patent Public Disclosure No. 83730/1984); and a chelate resin having functional groups of a specified structure in the molecule is used to recover dissolved metals by adsorption (Japanese Patent Public Disclosure No. 11224/1985).
To date, the conventional methods of recovering uranium from seawater using adsorbents or those for recovering dissolved metals using chelate resins have not been implemented in practice since they are incapable of cost-effective collection of uranium and other rare metals. However, for Japan which is by no means rich in mineral resources, it has been long desired to exploit the metals that are dissolved in the surrounding sea.
(a) The present has been accomplished with a view to developing a material that is strong enough to withstand hostile weather conditions in ocean and which has high performance in collecting dissolved metals from seawater. To attain this object, the fiber of a polyolefin such as polyethylene or polypropylene that is a highly durable polymer is used as a substrate, side chains are grafted to the substrate polymer by radiation-initiated graft polymerization, and then an amidoxime group and a hydrophilic group are introduced into the same graft side chains.
The collector of the invention is produced by a process comprising the following steps: (1) to generate a reaction initiating species (radicals), a substrate comprising the fiber of a polyolefin such as polyethylene or polypropylene is exposed to electrons; (2) grafting a polymerizable monomer having a cyano group such as acrylonitrile (CH2xe2x95x90CHCN) onto the polyolefin fiber in the presence of a polymerizable monomer having a hydrophilic group; and (3) then reacting the cyano groups in the graft side chains with hydroxylamine (NH2OH) to convert them to amidoxime groups, whereby both amidoxime and hydrophilic groups are introduced into the same graft side chains.
If desired, grafting of the polymerizable monomer having a cyano group onto the polyolefin fiber in the presence of the polymerizable monomer having a hydrophilic group is performed at a properly adjusted molar ratio of the two polymerizable monomers and, thereafter, the cyano groups in the graft side chains are reacted with hydroxylamine (NH2OH) to be converted to amidoxime groups, whereby the amidoxime and hydrophilic groups are introduced at a molar ratio of 70:30-30:70, preferably 60:40-40:60, more preferably 50:50.
(b) The collector of the invention needs only to be submerged and anchored in seawater such as the Kuroshio current so that slightly dissolved useful metals such as vanadium, uranium, cobalt, titanium and molybdenum are efficiently recovered from the seawater. The collector is characterized in that the fiber of a polyolefin such as polyethylene that is commonly used in oil fences is irradiated to introduce a chemical structure capable of selective trapping of metals.
Specifically, the collector of the invention can be produced by one of the following methods: (a) acrylonitrile is grafted to a substrate fiber in the form of either a nonwoven or woven cloth of the fiber of a polyethylene such as polypropylene or polyethylene and amidoxime groups are introduced into the graft side chains; (b) acrylonitrile and a polymerizable monomer having a hydrophilic group are co-grafted to the substrate fiber in either nonwoven or woven cloth form and amidoxime groups are introduced into the graft side chains made of acrylonitrile; and (c) the fiber of a polyolefin such as polypropylene is coated with a different polyolefin such as polyethylene to form a fiber of a core/sheath structure, a substrate is formed of this fiber in the form of either a nonwoven or woven cloth, either acrylonitrile or a polymerizable monomer having a hydrophilic group or both are grafted to the substrate fiber, and amidoxime groups are introduced into the graft side chains made of acrylonitrile.
For actual use, a plurality of such collectors are superposed and sandwiched between nets and a plurality of the resulting assemblies are stacked in position at suitable spacings to construct a cassette of collectors.
To collect dissolved metals from seawater, the cassette is placed in a number of corrosion-resistant cages, which are bound to a rope at suitable spacings; an anchor is attached to the submerged end of the rope and a buoy is attached to the other end so that the cassette is anchored in seawater either depthwise or laterally as long as the collectors are kept in contact with the seawater to collect dissolved metals from it.